Nanoscience for
Sustainable Energy
HEINER LINKE AND LARS SAMUELSON | NANOLUND
Nanoscience is the art of understanding and
using the new physical behavior exhibited by
very small material components – and it offers
a wealth of opportunities to help achieve a
sustainable energy supply. This is one of the aims
of NanoLund, the Center for Nanoscience at
Lund University with more than 200 scientists
at the faculties of engineering, science and
medicine. A special focus is on materials science
and applications of nanowires – thin needles
about 100 nanometers in diameter (a nanometer
is one billionth of a meter). The use of nanowires
allows us to achieve entirely new combinations
of materials that previously were impossible, and
allow, for example, for more efficient solar cells,
light emitting diodes and devices that convert
heat into electricity.
This pamphlet is an excerpt from ”Today’s
Great Explorers - New frontiers in strategic
research at Lund University”, edited by Sara
Naurin and published by Lund University in
December 2014.
NANOSCIENCE FOR SUSTAINABLE ENERGY
A GLOBALLY sustainable energy supply will be possible only
if we harvest many types of renewable energy sources (such as
wind, sunlight, waves and tides) in combination with reduced
energy consumption. In other words, we must find better and
more efficient ways of converting one form of energy into
another. Crucially, this must be done in a way that is sustainable
in the broadest meaning of the word: the novel techniques
must be economically affordable, must have no adverse impact
on environment and health, and must be achievable within the
scope of Earth’s natural resources, including its supply of raw
materials. Nanoscience – the art of understanding and using
the new physical behaviour exhibited by very small material
components – offers a wealth of opportunities to help achieve
a sustainable energy supply. Therefore, sustainable solutions
to more efficient energy conversion are naturally one of the
overarching aims of NanoLund, the Center for Nanoscience
at Lund University.
On the following pages we describe our activities in this area,
ranging from basic research all the way to technology that is
about to hit the market, and from education to collaboration
with industry. Using several specific examples, such as the use
of sunlight to generate electricity and efficient illumination
techniques based on light emitting diodes, we will describe how
nanostructures can make energy conversion more efficient,
while at the same time drastically reducing the amount of raw
materials needed. We will describe our efforts to ensure the
safe handling of nanomaterials, from their invention to their
use in products. And we will describe how we take a longterm view, developing entirely new physics concepts for energy
conversion at the smallest scale, which could lead to methods
for efficient energy conversion that are not known today and
that may be implemented decades from now.
NANOSCIENCE FOR SUSTAINABLE ENERGY
Nanowires: tiny needles with unique properties
When the size of the components of a material are reduced
below about a tenth of one micrometre (thousands of times
smaller than the diameter of a human hair) we can achieve
entirely new optical, electrical and mechanical properties.
For example, nanoparticles made of gold do not have the
characteristic shiny metal colour, but instead assume a gentle
red – a testament to the different way the nanoparticles interact
with visible light, compared to a larger “bulk” piece of gold.
For the past dozen or so years, scientists at Lund University
have been focusing on a specific type of nanomaterials called
nanowires. These are thin needles about 100 nanometres in
diameter (a nano¬metre is one billionth of a metre). The focus
in Lund is on semiconductor nanowires made from elements
from groups III and V of the periodic system. These materials
are already widely used for electronics and optoelectronics:
in mobile wireless networks, fibre optics, and light-emitting
diodes.
Nanowires are made in a way that is very similar to the
way crystals form naturally. By supplying the raw materials
(the elements from which the target crystal is made) at a
temperature and pressure close to where a crystal forms
spontaneously, nanowires can be nurtured to grow to the
desired length from a seed particle whose size determines the
resulting needle’s diameter. The wires of the highest quality
(with the best electrical and optical properties) are achieved
when supplying the growth elements in gas form. This is the
approach pioneered in Lund and a small number of other
places around the globe in the early 2000s, and perfected since
then. So why have more than a hundred scientists in Lund
spent over ten years focusing all of their attention on tiny
needles? A key reason is that the use of nanowires allows us to
achieve entirely new combinations of materials that previously
were impossible. This is important because all electronic and
NANOSCIENCE FOR SUSTAINABLE ENERGY
A regular array of nanowires, grown from seed particles (still visible
on top of the wires) that were positioned on the substrate using
nanofabrication techniques. The diameter of the wires is about 100
nanometres, more than a hundred times thinner than a human hair.
A regular array like this one can serve as antennas to effectively absorb
or emit light of a certain wavelength. The photograph was taken using
an electron microscope. Photo: Gustav Nylund, Lund University.
optoelectronic devices are enabled by effects that occur at
the interface between two materials. However, in traditional
electronics (where large plates of material, called wafers, are
processed in one piece) severe limitations are imposed by what
materials can be combined: both materials must have precisely
the same distance between their atoms, or else cracks will
form that destroy the electronic and optical properties. With
nanowires, we do not have this limitation: the wires are so thin
that cracks don’t propagate along the wire, or don’t even form
in the first place. As a consequence, we can use a much wider
range of semiconductor materials to be sandwiched together in
NANOSCIENCE FOR SUSTAINABLE ENERGY
nanowires, with superior electrical and optical properties. This
gives us the opportunity to construct better, higher performing
semiconductor devices for a very large range of applications,
ranging all the way from quantum mechanical devices to faster
electronics and advanced medical devices.
Furthermore, the small dimensions of nanowires are
comparable to the wavelength of visible light, and to the
wavelength (or size) of electrons. By carefully choosing the
dimensions of the wires and their components, and by creating
regular arrays of nanowires in highly controlled patterns, it
is possible to create completely new electronic and optical
properties. More energy with less material: nanowire solar cells
Solar cells (photovoltaic cells) are semiconductor devices that
capture light (photons) arriving from the sun, and use their
energy to push electrical charges through a wire – creating
electric power. The amount of energy available from the sun
is staggering; the light delivered by the sun to the surface of
the earth in one hour would be sufficient to cover humanity’s
energy needs for an entire year!
Why, then, do we not already use this almost inexhaustible
amount of available solar energy to cover most or all of our
electricity needs?
One answer is that we simply haven’t got round to it yet.
In fact, the amount of solar power conversion capacity
installed on fields and rooftops worldwide has been increasing
exponentially for the past 15 years, and solar power is here to
stay. However, for this trend to continue, and for photovoltaic
energy to grow into a truly substantial contributor to electricity
needs worldwide, we must further increase the efficiency at
which solar cells convert light into electricity. This is because
higher efficiency will reduce the amount of raw material
(currently mostly silicon) needed to produce the same amount
of photovoltaic power, and will thus reduce the cost and
NANOSCIENCE FOR SUSTAINABLE ENERGY
increase sustainability. Nanowires offer unique opportunities
to create more efficient and cost-effective solar cells. In 2013,
nanoscientists at Lund University and their international
collaboration partners attracted much attention by producing
the solar cell with the highest efficiency ever achieved using
nanowires, 13.8%. This may not sound very impressive
compared to the best commercial, wafer-based (planar)
silicon devices (23%) or to the record-high values of up to
44% achieved in research labs elsewhere for concentrator solar
cells. However, crucially, by using nanowires, the Lund team
achieved this efficiency with only 3% of the active material
that would have been needed with traditional techniques. This
dramatic reduction in material use, which offers great promise
for future cost reductions, was achieved by very carefully
positioning the nanowire solar cells in the form of an antenna
array, which absorbed the sunlight very effectively, such that
much less material was needed to capture the light.
This is only the beginning for nanowire-based solar cells.
Advanced solar cells, such as those planar cells that achieve
conversion efficiencies above 40%, use several layers of
material, each converting one part of the solar spectrum into
electricity. Such multi-junction solar cells are the next step also
for nanowires, which will be helped by the fact that nanowires
are much more suited for sandwiching materials on top of one
another.
Aerotaxy: flying nanowires boost traditional solar
cells
The techniques described above still have one major
drawback: they rely on traditional crystal growth methods,
where nanowires can be only grown one layer at a time and
on expensive substrates. We have recently demonstrated a
radically new method of manufacturing nanowires, offering
unprecedented growth speeds and material production
NANOSCIENCE FOR SUSTAINABLE ENERGY
capacity. In this growth method, nanowires grow as an aerosol
(in the form of particles “flying” in a stream of gas) without
the use of any substrate. The method has been named aerotaxy.
Using aerotaxy, wires can be grown in a continuous flow in
large quantities, and one vision is to simply add a layer of
nanowires on top of traditional silicon solar cells, boosting
their efficiency by enabling them to more efficiently access
a wider part of the solar spectrum. Fundamental studies of
aerotaxy are presently funded by a grant from the Knut and
Alice Wallenberg Foundation, and the technique is being
upscaled for pilot production by Sol Voltaics AB in Lund.
Pleasing the eye: LEDs made for humans
General lighting – street lamps, household and factory
lighting – today consumes as much as 20–25% of the
developed world’s electric power. One reason for this is that
traditional incandescent light bulbs and halogen lamps are
very inefficient, since they produce much more heat than
light. Even the use of the more efficient compact fluorescent
lamps is unsustainable, since they contain the environmentally
hazardous element mercury. The most energy-efficient lighting
technology available is light-emitting diodes (LEDs). Today,
white LED lamps have efficiencies of 30–50% in converting
electrical power to light, which is ten times higher than
traditional incandescent lamps. This is currently achieved by
using a blue-emitting LED that is used to “pump” a phosphor
material that emits in the green and red spectral regions, on
average seen by the human eye as “white” light. One major
challenge and opportunity for the future will be to directly
create the three basic colours, i.e. red, green and blue (RGB),
with LEDs, which would increase efficiency and long-term
stability. Furthermore, light produced by combining the three
LED sources can be “mixed” to tune the colour temperature
to optimally match the preferences of the human eye, and
NANOSCIENCE FOR SUSTAINABLE ENERGY
adjusted for use in specific work or social environments. The
nanowire technology developed in our laboratory enables
the best blue and green LEDs for such lighting applications,
and we are currently developing efficient red nanowire LEDs,
something that would be impossible with traditional planar
LED technology. The Lund technology for nanowire-based
LEDs has been transferred to the spin-off company Glo AB,
which currently has more than 60 employees and is pursuing
pilot production of perfectly mixed RGB light for displays as
well as for lighting applications.
Electricity from heat
If you take a piece of electric conductor, such as a semiconductor,
and heat one end while keeping the other cold, a small electric
current can be generated. This effect, called thermoelectricity,
can be used to convert heat directly into electric power,
without a turbine or other mechanical device. Thermoelectric
devices could thus be used to extract useful electricity from
heat sources that now go to waste: for example from the heat
emit ted from cars’ exhaust pipes, or from the temperature
difference between a residential gas burner and the water for
the shower.
However, current thermoelectric devices are not efficient
enough to make their use in these applications worthwhile.
Furthermore, some of the most efficient thermoelectric
materials available are rare or toxic (or both!), making their
large-scale use unsustainable.
Semiconductor nanowires have unique properties that make
them very interesting candidates for future, efficient thermalto-electrical energy converters. Firstly, their small diameter
effectively blocks heat flow, reducing heat loss and making
them more efficient. Secondly, their electronic properties
– which determine the amount of electric power they can
NANOSCIENCE FOR SUSTAINABLE ENERGY
produce from a given amount of heat – can also be improved
by tuning the wire diameter. At least, this was predicted more
than 20 years ago by MIT professor Mildred Dresselhaus, but,
until recently, had never been realised in practice.
Lund researchers very recently demonstrated, for the first time,
that nanowires indeed can produce more power from heat
than the same material in bulk form. At the moment, this has
only been shown at very low temperatures where such devices
are not useful for electric power generation. Nevertheless, the
breakthrough sets the stage for future, efficient nanowirebased
thermoelectric converters in your car or home.
Safety: is “nano” always good?
Nanoscience is about understanding and eventually taking
advantage of the new properties a material exhibits on the
nanoscale, for example in the form of nanoparticles or
nanowires. However, there is no guarantee that these novel
properties are always exclusively positive. In fact, it is well
known that some types of nanoparticles, when inhaled or
ingested, can be hazardous to human health. A number of
factors can play a role: for example, the particle’s catalytic
properties, which can be much enhanced by its large surfaceto-volume ratio, its size and shape (making it difficult for the
cells of the immune system to absorb the particle) or other
properties. Just as for any new chemical, it is very important
to evaluate a nanomaterial’s safety before it is used in the lab
or in products, and to consider exposure risks to workers and
customers throughout a product’s life cycle. This applies to
solar cells or LEDs as much as for any other nanomaterialbased device or product.
The exposure routes (the ways by which humans or other
organisms may touch, inhale or ingest a nanoparticle) of
nanomaterials are different from those of bulk materials.
NANOSCIENCE FOR SUSTAINABLE ENERGY
New methods need to be developed to measure and evaluate
health and environmental risks during production and use of
nanoparticles.
Within the Lund nanoscience environment, more than 20
scientists with backgrounds in occupational health, physics and
chemistry combine their expertise to evaluate safety aspects of
nanoparticles in general and of nanowires in particular. This
includes the development of methods to accurately measure,
for example, the quantity of airborne nanowires that may end
up in the lungs of workers in university laboratories as well
as in start-up companies that commercialise nanowirebased
technology.
Novel concepts
The uses of nanowires described above primarily concern
improvements to technology that existed long before the
advent of nanowires. Could nanoscience also offer entirely new
routes to efficient energy use that have never been tried before?
We be lieve that the answer is “yes”, and have initiated several
fundamental science thrusts to explore such possibilities.
One example is the use of molecular devices for energy
conversion. The prime example of such systems can be found
in biology, where molecular motors (complex protein-based
molecules) perform useful tasks such as intracellular transport.
To do this, molecular motors convert chemical energy directly
into mechanical work with higher efficiency than even the
most sophisticated car engine. To learn from nature, we
construct artificial molecular motors and explore synthetic
nanodevices powered by biological molecular motors. As
part of a collaboration financed by the European Union, we
explore computing devices that use molecular motors instead
of electrons, and that use a hundred times less energy than
traditional electronic computers. We also study forms of energy
NANOSCIENCE FOR SUSTAINABLE ENERGY
conversion that are entirely different from anything that is
used in existing technology. For example, we are learning how
information can be transformed into useful energy in devices
that can “observe” and take advantage of fluctuations of a small
object such as an electron or molecule. This work involves a
reformulation of thermodynamics on the nanoscale, since the
classical thermodynamic theory was developed specifically for
large-scale steam turbines or car engines, and cannot be used
to understand energy conversion on the tiny scale of single
molecules.
Looking ahead, there are many more opportunities for Lund
nanoscience to address society’s grand challenges, leading to a
better life for many. In one focus area we will target Nanoscience
for the benefit of the developing world. Current research will help
provide affordable access to both solar power and efficient LED
lighting in the developing world, and will thus help improve
living conditions and support children’s education. Further
development of LEDs will allow us to efficiently produce
deep-ultraviolet light to kill microorganisms, for use in water
purification systems. Lund micro- and nanotechnology can
also be used to develop cheap, hand-held devices for quick
diagnostics for use in remote health clinics.
We also foresee the use of nanowire probes in neuroscience and
in cancer and diabetes research and treatment, in collaboration
with the strong Lund-based research efforts in these areas
(including the Neuronano Research Center, EXODIAB and
BioCARE). The ability to probe, manipulate and monitor one
or many individual cells may help answer important scientific
questions in these fields.
NANOSCIENCE FOR SUSTAINABLE ENERGY
Fundamental and applied science, education and
innovation
The science described above is possible only in a highly
integrated research environment that spans basic and applied
research, materials science and physics, chemistry and biology,
technology, and medicine. It is our mission to do both: focus
on addressing important problems of today, while also allowing
blue-sky research to thrive and provide novel approaches to
solving the problems of tomorrow.
One of our most important roles is the education of
nanoscientists for careers in academic research as well as in
industry, with the ability to integrate knowledge and skills
from many disciplines, and with a deep understanding of the
need to apply this expertise safely and sustainably. We do this
in our flagship degree programme Engineering Nanoscience,
as well as in the education of the approximately 100 doctoral
students engaged in Lund nanoscience. Currently, we are
developing an internship programme with the aim of offering
our doctoral students the opportunity to gain industrial work
experience for a few months during their studies.
However, to turn ideas into commercial technology that
can improve our lives, great interdisciplinary science and
bright young people alone are not enough: it is a long road
from basic research to a profitable product. Working jointly
with regional policy-makers, research institutions and
entrepreneurs, we are therefore currently planning for an
expansion of the fundamental research facilities towards nanoproduction. It is our vision to ultimately create a complete
innovation ecosystem, comprising all steps in the innovation
production chain, using resources jointly operated and owned
by academia and industry. This vision includes ProNano, a
pilot production facility that will preferably be located in the
Science Village area close to the MAX IV and ESS facilities,
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with much of the necessary infrastructure and support
facilities needed for industrial development. In this way, we
will be able to vertically integrate unique competences all the
way from education and basic research, via applied R&D
and pilot facility establishment, to enable the development
of an advanced industrial infrastructure for nanomaterials
production. Our research can thus reach its full potential in
terms of its benefit to society. ¨
Lars Samuelson initiated the
Nanometer Structure Consortium in 1988, shortly after being
recruited to Lund as a young
Professor of Semiconductor
Electronics. In the late 1990s he
focused the consortium’s work
on nanowires. He is the founder
of several spin-off companies, including Glo AB, which
commercialises nanowire-based
light-emitting diodes (LEDs).
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About Nanoscience at Lund University
Founded in 1988 as the Nanometer Structure Consortium (nmC), NanoLund is
today Sweden’s largest research environment for nanoscience, engaging more
than 240 scientists and PhD students at the faculties of engineering, science
and medicine at Lund University. Its vision is to bring together the most creative
minds in a worldleading interdisciplinary research environment to help overcome
society’s grand challenges by pushing the frontiers of nanoscience.
To support these strategic aims, we provide multiple platforms for interdisciplinary exchange and collaboration across faculties and continuously
develop joint, coherent goals. We fund strategically important projects,
develop and maintain joint infrastructure for academic and industry users, and
offer education in nanoscience at all levels, including a five-year programme
in Engineering Nanoscience that is closely integrated with our research. Our
effective innovation pathways have created several spin-off companies that
currently employ more than 150 people.
Nanoscience at Lund University changed name to NanoLund in June 2015
www.nano.lu.se
Heiner Linke is a Professor of
Nanophysics at Lund University
and the director of NanoLund
since 2013. He joined Lund University in 2009, following eight
years at the physics department
and Materials Science Institute
of the University of Oregon.
His research focuses on energy
conversion at the nanoscale
in both semiconductors and
biomolecular systems.
Print Media-Tryck | Lunds universitet 2015
LUND UNIVERSITY
NanoLund
Box 118
221 00 Lund
Sweden
www.nano.lu.se